User interfaces that are used to translate human movements to machine movements are used in myriad industries. For example, some aircraft flight control systems include a user interface in the form of one or more control sticks, pedals, or other mechanisms. The flight control system, in response to input forces supplied to the user interface(s) from the pilot and/or co-pilot, controls the movements of various aircraft flight control surfaces. No matter the particular end-use system, the user interface preferably includes some type of mechanism to supply haptic feedback, through the user interface, to the user.
Many haptic feedback mechanisms are implemented using a force sensor as the primary input device to the feedback loop. In most instances, the force sensor drives some type of servo amplifier, which in turn drives a motor. The motor, which may be coupled to the user interface via a gearbox, supplies a feedback force to the user interface. Although these types of haptic feedback mechanisms are generally safe and reliable, they do suffer certain drawbacks. For example, the force sensor (or sensors) can increase overall system cost and complexity, and when redundancy is employed to increase overall system reliability, this cost and complexity can be significant.
In addition to increased costs, the force sensor many times senses undesired high frequency vibratory force inputs from the human hand. These force inputs, when sensed, may be amplified, and tuning the feedback loop to reject these vibratory force inputs can adversely impact system characteristics. Moreover, the servo feedback loop can be difficult to tune for acceptable feel because of the high gain associated with a force sensor, and because the motor may be separated from the force sensor by the gearbox. As a result, in some designs additional sensors may be used to sense motor velocity and/or angular acceleration, further adding to costs.
To overcome at least some of the above-noted drawbacks associated with the use of force sensors, some haptic feedback mechanisms use one or more position sensors rather than force sensors. However, presently used position sensors do not overcome all of the potential drawbacks associated with force sensors. Most significantly, high-accuracy position sensors can increase overall system costs. This can be especially true when there is a need to accurately determine user interface displacement in two axes.
Hence, there is a need for a system that can determine the displacement of, for example, a user interface in two axes with relatively high accuracy and at a relatively low cost. The present invention addresses one or more of these needs.